Wavefront sensors are used to provide image control in, for example, active optical systems used in various devices such as satellites, telescopes, and cameras. More particularly, known wavefront sensors include elements which provide information about phase distortions in a received wavefront, and/or which analyze, measure, and provide information signals to correct for aberrations (phase distortions) received in optical wavefronts. Such phase distortions or aberrations are produced in an image by, for example, a thermal expansion or contraction of a structure in, for example, a telescope or camera due to varying temperatures which changes distances between lenses and/or mirrors therein. It is to be understood that the wavefront sensor is a first element of an active control system which includes three elements. The second element is a controller which processes signal information generated by the wavefront sensor using an appropriate algorithm (software device) to analyze and determine what should be done with the signal information from the wavefront sensor. The third element actually actively provides a change to correct a problem (e.g., defocussing) detected by the controller from the signal information of the wavefront sensor. In this regard see, for example, the article by Peter A. Jones entitled "Alignment And Focus Control Of A Telescope Using Image Sharpening", published in the SPIE Proceedings. Vol. 1542-18, 1991, at pages 194-204, which discusses a thought process for using information from a wavefront sensor in active optical systems.
Various types of wavefront sensors are known in the prior art. For example, it is known in radar, sonar, and camera focussing technology to send out a signal wavefront and measure the round trip time of such signal wavefront to determine distance to an object. Such distance information is then used to focus the camera or determine where an object is located in object space.
U.S. Pat. No. 4,667,103 (J. T. Watson et al.), issued on May 19, 1987, discloses a wavefront sensor for a large optical system, such as a telescope, comprising an objective lens, a reticle with at least one slit therein to provide an aperture, a field lens, and an array of infrared sensor cells. The objective lens images an infrared ground scene onto the reticle which provides image signatures for various portions of the aperture to an array of infrared sensor cells via the field lens. The resulting time difference between the image signatures of the aperture represents relative image displacement caused by a wavefront slope variation in the actual received wavefront.
U.S. Pat. No. 4,712,913 (N. Bareket), issued on Dec. 15, 1987, discloses a linear-scanned-array wavefront sensor. In the wavefront sensor, a plurality of sample beam wavefronts derived from selected portions of a wide aperture optical beam wavefront enter an entrance pupil located adjacent a focal surface on which the sample beams are individually focussed. A relay lens system transmits the sample beams to a beam splitter which divides each of the sample beams into an undeviated and a deviated component. The undeviated components of the sample beams image the entrance pupil onto a first dithering scanning mirror, and the deviated components of the sample beams image the entrance pupil onto a second dithering scanning mirror. The first and second scanning mirrors are dithered to sweep sample beam spots across first and second focal planes, respectively, where a plurality of linear photodetector arrays are positioned. Line centroids of the sample beam spots at the first and second focal planes are calculated by a processor. Any deviation in the line centroid provides an indication of an aberration in the received wavefront.
Prior art wavefront sensors for use with active optical systems require moving parts and many components such as reference lasers, lenses, gratings, beam splitters, stationary and movable mirrors, multiple light beam paths, and a plurality of detector arrays which make such wavefront sensors complicated and expensive. Still further, many prior art wavefront sensors use a point source, as a star or a reference laser, to determine whether an out-of focus condition exists, and are unable to provide such information when looking at a wide area (extended scene) in object space.
It is desirable to provide a wavefront sensor which is simple and relatively inexpensive, and provides output signals from which information relating to aberrations in wavefronts received directly from an extended scene in object space can be determined and, in turn, used by separate active devices to correct for such aberrations.